The present invention relates to a passive in-line adsorber system capable of meeting the ultra-low emission vehicle (ULEV) standards by utilizing a combination of light-off catalyst, a burn-off catalyst, a hydrocarbon adsorber, and optionally, a three-way catalyst as a main converter.
Internal combustion engines emit large amounts of unburned hydrocarbons during the cold start of an engine due to the rich fuel mixture used in such engines, and the necessarily incomplete combustion at start-up. This emission of unburned hydrocarbons continues until the main catalyst reaches its "light-off" temperature, at which point the catalyst begins to convert the hydrocarbons to harmless gases. It has been determined that the typical light-off times for most internal combustion engines are around 50 to 120 seconds, during which time significant amounts of hydrocarbons are emitted into the atmosphere. The actual light-off time for any system will depend on the position of the catalyst relative to the engine, as well as the noble metal loading.
While catalytic converters are well known for reducing oxides of nitrogen (NOx), and oxidizing hydrocarbons and carbon monoxide from automobile exhaust, these reactions typically take place at temperatures of 300.degree. C. or higher. However, to increase the effectiveness of automotive emission control systems, at much lower temperature, that is, during cold start, a significant amount of hydrocarbons must be adsorbed and held until the converter attains its light-off temperature.
In the past, attempts have been made to improve efficiency using complex and expensive valving systems, or by using multiple catalysts placed in series along the exhaust stream path. Others have attempted to deal with the above problems by splitting the exhaust stream into equal portions which are directed to different converters or filters. Still, others have disclosed a multiple catalytic converter system in which flow of the exhaust gas is controlled to each converter using a pressure sensitive flow control valve.
A system has also been disclosed in which the amount of hydrocarbons entrained in exhaust gas is reduced at low temperatures and during low engine load conditions by passing the exhaust gas through a catalytic bed of sufficient volume having low space velocities to ensure that the exhaust gas remains in contact with the catalyst bed for a predetermined period of time to ensure adequate conversion. At high engine temperatures and high engine loads, the gas is passed through a small volume catalytic converter of high space velocities to allow only minimal contact with catalyst bed. The control mechanism for directing the exhaust gas flow includes a valve and an electronic control system for controlling the valve. None of the above systems provide for purification by adsorbing and desorbing the hydrocarbons at appropriate intervals.
Recently, several methods have been proposed for trapping or removing these unburned hydrocarbons, including zeolite traps, carbon traps and electrically heated catalysts. For example, a system has been disclosed which includes a hydrocarbon adsorbing material made up of a substrate coated with mixed powders of ZSM-5 zeolite ion-exchanged with Cu and Pd, and placed inline and upstream from a Pt/Rh catalyst.
More recently, co-pending, co-assigned application Ser. Nos. 08/106,512 and 08/106,514 have disclosed novel approaches for improving the capacity of zeolites to adsorb a broad range of hydrocarbons using modified zeolites. For the most part, many of the above systems rely on complex valving systems for exhaust gas flow. Since the effectiveness of the catalytic converter, at least with respect to hydrocarbon conversion varies with engine operating conditions such as temperature and engine speed, there continues to be a need for engine exhaust systems which are capable to adjusting effectiveness based on prevailing engine operating conditions.
Accordingly, it is the object of the present invention to provide a better and improved engine exhaust system for reducing or removing unburned hydrocarbons from exhaust emissions, particularly during cold-start.